Method of fabricating a group III-V semiconductor light emitting device with reduced piezoelectric fields and increased efficiency

ABSTRACT

An optical semiconductor device having a plurality of GaN-based semiconductor layers containing a strained quantum well layer in which the strained quantum well layer has a piezoelectric field that depends on the orientation of the strained quantum well layer when the quantum layer is grown. In the present invention, the strained quantum well layer is grown with an orientation at which the piezoelectric field is less than the maximum value of the piezoelectric field strength as a function of the orientation. In devices having GaN-based semiconductor layers with a wurtzite crystal structure, the growth orientation of the strained quantum well layer is tilted at least 1° from the {0001} direction of the wurtzite crystal structure. In devices having GaN-based semiconductor layers with a zincblende crystal structure, the growth orientation of the strained quantum well layer is tilted at least 1° from the {111} direction of the zincblende crystal structure. In the preferred embodiment of the present invention, the growth orientation is chosen to minimize the piezoelectric field in the strained quantum well layer.

FIELD OF THE INVENTION

[0001] The present invention relates to optical semiconductor devices,and particularly, to a structure for improving the efficiency of lightemitters and photodetectors fabricated from GaN-based semiconductors.

BACKGROUND OF THE INVENTION

[0002] In the following discussion a III-N semiconductor is asemiconductor having a Group III element and nitrogen. III-Nsemiconductors such as GaN are useful in fabricating light emittingelements that emit in the blue and violet regions of the opticalspectrum. These elements include light emitting diodes and laser diodes.Laser diodes that use semiconductor material based on GaN that emit inthe blue and violet regions of the spectrum hold the promise ofsubstantially improving the amount of information that can be stored onan optical disk. However, higher efficiencies are needed for bothsemiconductor light emitters and photodetectors. This is a particularlyurgent problem in GaN-based optical semiconductor devices using BN, AlN,GaN, or InN, which are compounds of nitrogen and Group III elements suchas B, Al, Ga, and In and their mixed crystal semiconductors(hereinafter, called GaN-based semiconductors).

[0003] Light emitting elements based on III-N semiconductors aretypically fabricated by creating a p-n diode structure having a lightgenerating region between the p-type and n-type layers. The diode isconstructed from layers of III-N semiconducting materials. After theappropriate layers are grown, electrodes are formed on the p-type andn-type layers to provide the electrical connections for driving thelight-emitting element.

[0004] One class of blue and green light-emitting diodes (LEDs) orshort-wavelength laser diodes (LDs) use GaInN/GaN strained quantum wellsor GaInN/GaInN strained quantum wells located between the n-type andp-type layers to generate light by the recombination of holes andelectrons injected from these layers. In prior art devices, a strainedGaN-based semiconductor layer is constructed by growing a {0001} planeof a normal GaN-based crystal. The resulting layer has a largepiezoelectric field. For example, in a Ga_(0.9)In_(0.1)N strained layer,an extremely large piezoelectric field of around 1 MV/cm is generated.

[0005] Usually, when an electric field exists in a quantum well, theenergy band of the quantum well layer tends to tilt substantially as theelectric field increases. As a result, the wave functions of theelectrons and holes separate from one another, and the overlap integralsof both wave functions decrease. Since the optical properties such asthe light emission and absorption efficiencies depend on these overlapintegrals, the efficiency of these devices decreases with increasingelectric fields.

[0006] Broadly, it is the object of the present invention to provide animproved III-N semiconductor device in which the efficiency of lightgeneration or detection is increased relative to prior art devices.

[0007] It is a further object of the present invention to provide astrained quantum well layer having a reduced piezoelectric field..

[0008] These and other objects of the present invention will becomeapparent to those skilled in the art from the following detaileddescription of the invention and the accompanying drawings.

SUMMARY OF THE INVENTION

[0009] The present invention is an optical semiconductor device having aplurality of GaN-based semiconductor layers containing a strainedquantum well layer in which the strained quantum well layer has apiezoelectric field that depends on the orientation of the strainedquantum well layer when the quantum layer is grown. In the presentinvention, the strained quantum well layer is grown with an orientationat which the piezoelectric field is less than the maximum value of thepiezoelectric field strength as a function of the orientation. Indevices having GaN-based semiconductor layers with a wurtzite crystalstructure, the growth orientation of the strained quantum well layer istilted at least 1° from the {0001} direction of the wurtzite crystalstructure. In devices having GaN-based semiconductor layers with azincblende crystal structure, the growth orientation of the strainedquantum well layer is tilted at least 1° from the {111} direction of thezincblende crystal structure. In the preferred embodiment of the presentinvention, the growth orientation is chosen to minimize thepiezoelectric field in the strained quantum well layer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 illustrates the crystal structure of a WZ-GaN-basedsemiconductor.

[0011]FIG. 2 is a graph of the piezoelectric field generated in thequantum well with respect to the growth orientation of the WZ-GaN-basedsemiconductor quantum well.

[0012]FIG. 3 illustrates the crystal structure of a ZB-GaN-basedsemiconductor.

[0013]FIG. 4 is a graph of the piezoelectric field strength generated inthe quantum well with respect to the first path shown in FIG. 3.

[0014]FIG. 5 is a cross-sectional view of an edge emitting laser diodeaccording to one embodiment of the present invention.

[0015]FIG. 6 is a graph of the relative light generation efficiency ofquantum wells in a semiconductor device of the present invention and aprior art semiconductor device as functions of the well width.

[0016]FIG. 7 is a cross-sectional view of an edge emitting laser diodeaccording to a second embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0017] The present invention is based on the observation that thepiezoelectric field in a strained quantum well layer depends on theorientation of the crystal structure of the quantum well layer, andhence, by controlling the facet orientation, the piezoelectric field canbe minimized. The manner in which this is accomplished may be moreeasily understood with reference to two types of strained quantum wellstructures, those based on a wurtzite crystal structure and those basedon a zincblende crystal structure.

[0018] Refer now to FIG. 1 which illustrates a wurtzite crystal GaN(WZ-GaN) structure 10. The piezoelectric field generated in a crystalhaving a facet orientation along arc 11 in FIG. 1 is shown in FIG. 2 asa function of the angle θ between the {0001} direction and the facetorientation. The data shown in FIG. 2 is for Ga_(0.9)In_(0.1)N strainedquantum well layers. The piezoelectric field reaches maxima in the{0001} direction or the {000-1} direction, and has three orientations atwhich the piezoelectric field is zero. The same result is obtained forother arcs, e.g., arc 12. That is, the piezoelectric field is uniquelydetermined by the difference in the angle between the {0001} directionand the facet orientation of the concerned plane, i.e, the piezoelectricfield is independent of φ.

[0019] Hence it is clear from FIG. 2 that there are three sets of planesfor which there is no piezoelectric field. For example, the planes at90° to the C-axis, i.e., the A-plane, {2-1-10}, the M plane {0-110},etc. The planes around 40° and 140° to the C-axis also provide planeswith a zero piezoelectric field, i.e., the R planes {2-1-14}, {01-12},etc.

[0020] The strength of the piezoelectric field depends on thecomposition of the GaInN strained quantum well layer. However, the planeorientations in which the field is zero are, at most, only slightlyaltered. In particular, the 90° facet orientation measured from the{0001} direction where the piezoelectric field becomes 0 does not dependon the ratio of Ga to In. The plane orientations corresponding to the40° and 140° orientations discussed above change by no more than amaximum of 5° from the 40° and 140° values determined for thecomposition shown in FIG. 2.

[0021] A similar analysis can be applied to other crystal structures.Consider a zincblende crystal structure GaN-based semiconductor layer,referred to as ZB-GaN in the following discussion. AZB-Ga_(0.9)In_(0.1)N strained quantum well layer can be formed on GaN ina manner analogous to the WZ-GaN-based semiconductor strained quantumwell layer discussed above. FIG. 3 shows the crystal structure 20 of theZB-GaN-based semiconductor. To simplify the discussion, the sphericalcoordinate system used with reference to FIG. 1 will also be used here.The radius vector has a polar angle θ measured from the {001} directionand a cone angle, φ, about the {001} direction. First and second pathshaving a constant azimuth angle φ are shown at 21 and 22.

[0022] Refer now to FIG. 4, which is a plot of the piezoelectric fieldin the strained quantum well layer with respect to the polar angle θ forvarious orientations of the strained quantum well layer on path 21. InFIG. 4, φ=45° and the {001} direction corresponds to θ=0°. The {111}direction corresponds to θ=54.7°, the {110} direction corresponds toθ=90°, and the {11-1} direction corresponds to θ=125.3°. It is clearfrom FIG. 4, that the piezoelectric field has maxima in the {111}direction (θ around 55°) and the {11-1} direction (θ around 125°). Moreimportantly, the piezoelectric field goes to zero for θ=0, 90°, and180°.

[0023] A similar analysis with respect to path 22 shows that thepiezoelectric field is essentially 0 for all points along this path.Path 22 corresponds to a Ga_(0.9)In_(0.1)N strained quantum well layerin which the growth orientation corresponds to θ and φ=90°. Hence, in astrained quantum well crystal of ZB-GaN-based semiconductor, almost nopiezoelectric field is generated in the strained quantum well layer thathas growth planes beginning in the {001} plane or {011} plane and afacet orientation angle θ on path 22. A similar result holds for planesthat are equivalent to these.

[0024] The manner in which the above-described observations are used inthe fabrication of a light emitter will now be explained with the aid ofFIG. 5 which is a cross-sectional view of a laser 30 according to thepresent invention. If the crystal growth orientation is excluded, thecomposition of each deposited layer is essentially that used in aconventional laser diode.

[0025] Laser 30 is constructed from a number of layers. An n-type GaNcontact layer 33, an n-type AlGaN cladding layer 34, a strained multiplequantum well layer 35, a p-type AlGaN cladding layer 36, and a p-typeGaN contact layer 37 are successively deposited on a substrate 31 whichis typically, sapphire, SiC, or GaN. An n-electrode 38 and a p-electrode39 are deposited as shown.

[0026] The strained multiple quantum well layer 35 is typicallyconstructed from GaInN/GaN or GaInN/GaInN. In a laser diode according tothe present invention, the layers of the quantum well are caused to growsuch that the piezoelectric field generated by the layers is negligible.In conventional laser diodes, the {0001} plane of a sapphire substrateis used to grow the various layers. As noted above, this leads to a highpiezoelectric field and poor efficiency.

[0027] As noted above, there are a number of planes for which thepiezoelectric field is substantially zero. One of these is utilized in alaser diode according to the present invention. The particular planewill depend on the type of crystal. For example, in the case of a WZ-GaNlight emitter, the {2-1-10} plane of the strained quantum layer materialcan be caused to grow by selecting the appropriate growing surface ofsubstrate 31. If the substrate is sapphire, the sapphire is cut suchthat the {01-12} plane is used for growing layer 33. In the case of SiC,the {2-1-10} plane is used. In the preferred embodiment of the presentinvention, SiC with a growth plane of {2-1-10} is preferred.

[0028] The relative efficiency of a laser diode according to the presentinvention and a conventional laser diode grown on the {0001} plane of asapphire substrate is shown in FIG. 6 as a function of the width of thequantum well. Curve A is the efficiency for the device discussed abovewith reference to FIG. 5, and curve B is the efficiency of theconventional device. It will be appreciated from this figure that thepresent invention provides a substantial improvement in the efficiencyof light generation.

[0029] The present invention may also be utilized to provide improvedperformance from photodetectors. Photodetectors fabricated by growingthe device on the {0001} plane of a sapphire substrate exhibit anefficiency and absorption band that depend on light intensity. Inparticular, the efficiency of conversion increases with light intensitywhile the useful wavelength range decreases,

[0030] In a photodetector according to the present invention, the deviceis grown on a substrate that results in little or no piezoelectric fieldin the strained quantum well layer. Hence, the increase in efficiencyand decrease in absorption band are substantially reduced or eliminated.In general, the growing technique for a photodetector is the same asthat used to construct a light emitter; however, thicker strainedquantum well layers are utilized to improve the absorption of theincident light.

[0031] It would be advantageous in many circumstances to utilize asapphire or SiC substrate in which the layers, except for strainedquantum wells, are grown on the {0001} plane, since substrates cut toprovide growth on a {0001} plane are commercially available. Refer nowto FIG. 7 which is a cross-sectional view of the optical semiconductordevice 50 according to another embodiment of the present invention inwhich only the layers related solely to light emission and absorptionhave the desired facet orientation. Device 50 is constructed by growingan n-type GaN contact layer 53 and an n-type Al GaN cladding layer 54 onthe {0001} plane orientation on the substrate 51 such as SiC or GaNbased on conventional technology. Next, by selective growing orselective etching, the {2-1-14} plane or {01-12} plane is formed. TheGaInN/GaN or GaInN/GaInN strained multiple quantum well layer 55 is thenformed by repeating the crystal growth.

[0032] Next, the remaining p-type AlGaN cladding layer 56 and the p-typeGaN contact layer 57 are successively deposited and formed. The p-typeAl GaN cladding layer 56 and the p-type GaN contact layer 57 change thecrystal structure back to that corresponding to the {0001} plane fromthe facet orientation of the well layer 55 and become layers withspecific thicknesses. The n-electrode 58 and the p-electrode 59 areformed as the electrodes on the n-type GaN contact layer 53 and thep-type GaN contact layer 57, respectively. The growing surfaces 55A, 55Bon both sides of the GaInN strained multiple quantum well layer 55 arethe {01-12} plane or the {2-1-14} plane. The p-type AlGaN cladding layer56 and the p-type GaN contact layer 57 become flat growing surfaces. Tosimplify the next process, it is advisable that they be several micronsthick. In the preferred embodiment of the present invention, an AlNbuffer layer 52 is grown on the substrate 51.

[0033] As noted above, the specific plane selected for growing thequantum well layer depends on the crystal type. In WZ-GaN-based opticalsemiconductor devices, the {0001} plane may be utilized, since thisplane has excellent crystal quality and generates almost nopiezoelectric field. For devices based on different compoundsemiconductors such as AlN, it can be shown that the piezoelectric fieldas a function of the facet orientation behaves similarly to thatdescribed above if the crystal type is the same. The orientationinclination, θ, for which the piezoelectric field of 0 may, however,change by as much as 10°.

[0034] Various modifications to the present invention will becomeapparent to those skilled in the art from the foregoing description andaccompanying drawings. Accordingly, the present invention is to belimited solely by the scope of the following claims.

What is claimed is:
 1. In an optical semiconductor device having aplurality of GaN-based semiconductor layers containing a strainedquantum well layer, said strained quantum well layer having apiezoelectric field therein having a field strength that depends on theorientation of said strained quantum well layer when said quantum layeris grown, the improvement comprising growing said strained quantum welllayer with an orientation at which said piezoelectric field is less thanthe maximum value of said piezoelectric field strength as a function ofsaid orientation.
 2. The optical semiconductor device of claim 1 whereinsaid GaN-based semiconductor layers have a wurtzite crystal structureand wherein said growth orientation of said strained quantum well layeris tilted at least 1° from the {0001} direction of said wurtzite crystalstructure.
 3. The optical semiconductor device of claim 2 wherein atleast one other layer of said GaN-based semiconductor has a growthorientation in the {0001} direction.
 4. The optical semiconductor deviceof claim 2 wherein said strained quantum well layer is tilted at 40°,90°, or 140° from said {0001} direction.
 5. The optical semiconductordevice of claim 1 wherein said GaN-based semiconductor layers have azincblende crystal structure and wherein said growth orientation of saidstrained quantum well layer is tilted at least 1° from the {111}direction of said zincblende crystal structure.
 6. The opticalsemiconductor device of claim 5 wherein at least one other layer of saidGaN-based semiconductor has a growth orientation in the {111} direction.7. A method for fabricating a GaN-based optical semiconductor device,said method comprising the steps of: growing a first semiconductor layeron a substrate, said first semiconductor layer being grown with a firstfacet orientation; altering the surface of said first semiconductorlayer that is not in contact with said substrate such that said alteredsurface provides a growth orientation having a second facet orientationfor a subsequent semiconductor layer grown thereon, said second facetorientation differing from said first facet orientation; and growing astrained quantum well layer on said altered surface.
 8. The method ofclaim 7 wherein said step of altering said surface of said firstsemiconductor layer comprises selectively etching said firstsemiconductor layer or selective diffusion of said first semiconductorlayer.
 9. The method of claim 7 further comprising the step of growing asecond semiconductor layer on said strained quantum well layer, saidsecond semiconductor layer being grown with a facet orientation equal tosaid first facet orientation.